March 2014 

NASA/TM–2014-218177 

Identification of Vehicle Health Assurance 

Related Trends 

Nipa Phojanamongkolkij

Langley Research Center, Hampton, Virginia 

Joni K. Evans

Analytical Mechanics Associates, Inc., Hampton, Virginia 

Lawrence C. Barr

Volpe National Transportation Systems Center, Cambridge, Massachusetts 

Karen M. Leone

Vantage Partners, LLC, Brook Park, Ohio 

Mary S. Reveley

Glenn Research Center, Cleveland, Ohio 
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March 2014 

NASA/TM–2014-218177 

Identification of Vehicle Health Assurance 

Related Trends 

Nipa Phojanamongkolkij

Langley Research Center, Hampton, Virginia 

Joni K. Evans

Analytical Mechanics Associates, Inc., Hampton, Virginia 

Lawrence C. Barr

Volpe National Transportation Systems Center, Cambridge, Massachusetts 

Karen M. Leone

Vantage Partners, LLC, Brook Park, Ohio 

Mary S. Reveley

Glenn Research Center, Cleveland, Ohio 
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Table of Contents 

1. INTRODUCTION......................................................................................................................1 

1.1. Purpose of Study ...................................................................................................................1 

1.2. Overview of Study Contents ................................................................................................4 

2. SUMMARY OF NASA STATISTICAL STUDIES ...............................................................4 

2.1. Analysis of NTSB/FAA Data ...............................................................................................4 

 2.1.1. System/Component Failures/Malfunctions by Operation Category ..............................6 

 2.1.2. System/Component Failures/Malfunctions by Aircraft Category ................................14 

 2.2. Analysis of ASRS Data .....................................................................................................24 

3. REVIEW OF VEHICLE HEALTH MANAGEMENT (VHM) ISSUES AND FUTURE 

RESEARCH NEEDS ..................................................................................................................33 

3.1. CAST Safety Enhancements Reserved for Future Implementation (SERFIs) ....................33 

3.2. NTSB Most-Wanted List ...................................................................................................35 

3.3. NRC Decadal Survey of Civil Aeronautics ........................................................................36 

3.4. Future Aviation Safety Team Areas of Change .................................................................37 

3.5. Naval Air Systems Command (NAVAIR) ..........................................................................46 

3.6. Industry ...............................................................................................................................46 

 4. DISCUSSION AND CONCLUSIONS .................................................................................47 

ACRONYMS ................................................................................................................................50 

ACKNOWLEDGEMENT ..........................................................................................................50 

REFERENCES .............................................................................................................................51 

iii
1 

1. INTRODUCTION 

1.1. Purpose of Study 

The goal of the Vehicle Systems Safety Technology (VSST) Project of the NASA’s Aviation 

Safety Program (AvSP) is to “enable a reduction in accidents and incidents through enhanced 

vehicle design, structure, systems, and operating concepts” (Ref. 1). The VSST has three 

technical challenges contributing to this goal: 

 TC1: Improve Crew Decision-Making and Response in Complex Situations (CDM) 

TC2: Maintain Vehicle Safety between Major Inspections (MVS) 

TC3: Assure Safe and Effective Aircraft Control under Hazardous Conditions (ASC) 

The VSST management team uses systems analysis (1) “to provide independent information 

regarding the projected impact of the VSST research portfolio on its aviation safety goals”; and 

(2) “to identify key issues and maintain a research portfolio leading to potential solutions to the 

three challenges” (Ref. 1). To assist the management in achieving these objectives, several 

systems analysis milestones have been specified in the VSST project plan. The systems analysis 

milestone for fiscal year (FY) 2014 is focused on the MVS Technical Challenge. The goal of 

MVS is to “develop and demonstrate new integrated health management and failure prevention 

technologies to assure the integrity of vehicle systems between major inspection intervals and 

maintain vehicle state awareness during flight” (Ref. 1). The expected research outcome of the 

MVS TC is the set of six research products listed in Table 1. 

 
2 

Table 1. MVS List of Research Products 

MAINTAIN VEHICLE SAFETY BETWEEN MAJOR INSPECTION (MVS) 

MVS-1.1 Hybrid Structural Damage 

Diagnosis

Assessment of airframe structural health via 

sensors coupled with rapid large area inspection 

methods. 

MVS-1.2 Integrated Sensing and 

Healing System (ISHS)

Integration of self-healing materials that enable 

early detection of damage precursors, and 

increase durability and damage tolerance of 

airframes.

MVS-2.1 Vehicle Integrated 

Propulsion Research (VIPR)

Technologies developed under this element 

diagnose and monitor propulsion system in realtime, and mitigate potential issues through easily 

integrated, small, and low weight sensors 

avoiding costly retrofits while maintaining safety. 

MVS-2.2 Mitigating Turbomachinery 

Structural Failure

This technology will enable advanced inspection 

methods for recently implemented engine 

material technologies as well as robust engine 

material design methods for future emerging 

technologies.

MVS-3.1 Vehicle-Level Diagnostics 

and Integration

Integration of diagnostic data to provide an 

overall assessment of the vehicle state to identify 

potential maintenance and safety issues between 

inspections.

MVS-3.2 Physics-Based Models and 

Algorithms for Wiring Fault 

Detection

Combination of physics-based model and 

probabilistic fault detection algorithm to diagnose 

and identify chafed wires and degraded 

connectors for electrical wiring and interconnect 

systems (EWIS).

As detailed in the VSST project plan (Ref. 1), the MVS TC is described as follows: 

“MVS research centers around preventing vehicle failures, as well as quickly detecting and 

containing them when they do occur. MVS addresses critical risks for maintaining vehicle safety. 

The greatest amount of information about an airplane’s current health is currently obtained 

during major inspections. These inspections are thorough, costly, and are performed at set 

intervals based on fleet-wide averages for system and component reliability. There is a generous 

safety margin built into these intervals, but there are occasions where problems can come up 

during operations that were undetected at the last major inspection. MVS works to provide 
3 

information on potential safety-related systems problems to support in-flight decision-making 

and targeted maintenance that can address these problems. It accomplishes this goal through 

integrated systems consisting of high-capability sensors and diagnostic algorithms. It also 

develops capabilities to help preclude some of the most critical failures that can arise.” 

A summary of the goals, objectives, and products for the MVS TC are graphically depicted using 

an objectives tree format (Ref. 2) in Figure 1. 

Figure 1. MVS Objectives Tree 

KEY 

Will help to achieve 

To prevent vehicle failures, and quickly detect and contain them when they do occur for vehicle level 

and for the top three subsystems (engine, airframe, and avionics) involved in the aircraft fatalities.

To develop advanced materials and material 

coatings preventing damage from initiating and 

progressing for engine and airframe subsystems 

To improve inspection methods and on-board 

monitoring system for faults and failures at 

vehicle level and three subsystems 

MVS-1.2 

Integrated Sensing 

and Healing System 

(ISHS) 

MVS-2.2 

Mitigating 

Turbomachinery 

Structural Failure 

MVS-2.1 

Vehicle Integrated 

Propulsion Research 

(VIPR) 

MVS-1.1 

Hybrid Structural 

Damage Diagnosis

MVS-3.1 

Vehicle-Level 

Diagnostics and 

Integration 

MVS-3.2 

Physics-Based Models and 

Algorithms for Wiring 

Fault Detection 

GOAL 

OBJECTIVE 

PRODUCT 
4 

The specific MVS related FY14 systems analysis milestone is stated below: 

1) Deliver analysis of trends in aviation as related to vehicle health management (VHM) by 

reviewing the most current statistical and prognostics data available about accidents and 

incidents related to VHM areas. 

2) Deliver assessment of future directions in aviation technology related to VHM research 

areas through review of literature from academia, industry and other government 

agencies to establish requirements for future work in VHM. 

1.2. Overview of Study Contents 

The outcomes of this study are addressed in the next sections and in sequential order. Outcome 1 

is given in Section 2, which contains a summary of statistical analyses of accident and incident 

data that has been conducted by NASA researchers. Outcome 2 is addressed in Section 3, which 

is a summary of vehicle health assurance issues and future research needs that were derived from 

literature reviews, databases, and aviation subject-matter experts. Finally, Section 4 discusses 

and concludes the study. 

2. SUMMARY OF NASA STATISTICAL STUDIES 

Systems analysis personnel within the NASA AvSP have recently conducted statistical analyses 

of accident and incident data that are related to the MVS Technical Challenge. This section 

summarizes the results of these analyses. 

2.1. Analysis of NTSB/FAA Data 

A recent study has been conducted using accident data from the National Transportation Safety 

Board (NTSB) Aviation Accident and Incident Data System (restricted to 1996-2010), and 

incident data from the Federal Aviation Administration (FAA) Accident/Incident Data System 

(restricted to 1996-2010), especially those related to system/component failures/malfunctions 

(SCFM). 

The information the NTSB investigators collect during their investigations of these aviation 

events resides in the NTSB Aviation Accident and Incident Data System. A copy of this database 

in Microsoft Access format was obtained from the Aviation Safety Information Analysis and 

Sharing (ASIAS) department of the FAA’s Office of Aviation Safety in June 2012. At that point 

in time, the NTSB investigation was not complete for a substantial number of 2011 accidents, 

particularly those which occurred toward the end of the year. For this reason, all work on the 

database was restricted to 1986-2010, which was primarily an update of two years beyond the 

previous working version of the data the systems analysis personnel maintain. The update 

process requires several months of cross-checking various data elements and attempting to fill in 

any missing data, followed by the assignment of occurrence categories to each accident. 
5 

The NTSB database includes events involving a wide variety of aircraft (airplanes, helicopters, 

hot air balloons, gliders, ultralight, etc.) with operations conducted under various Federal 

Aviation Regulations (Part 91: General Aviation, Part 121: Commercial Air Carriers, Part 129: 

Foreign Air Carriers, Part 135: Commuters and On-Demand Air Taxis, Part 137: Agricultural 

Operations, etc.). The NTSB considers each event to be either an accident or an incident, with 

their definitions defined in Ref. 3. The NTSB does not investigate all incidents, but incidents as 

well as accidents are reported to the FAA by pilots, airport personnel and private citizens. 

The FAA maintains a database with the information that they receive in these reports and collect 

in their investigations. A copy of the FAA’s Accident/Incident Data System (AIDS) was 

obtained from Aviation Safety Information Analysis and Sharing (ASIAS) in July of 2011, 

which was late enough in the year that nearly all incidents from 2010 had been investigated. The 

current working copy of the AIDS database includes incidents from 1985-2010. A recent 

separate analysis showed substantial differences in several accident characteristics in the data 

prior to 1996. As such, for this analysis the decision was made to select the most recently 

available fifteen years’ worth of data (1996-2010). 

In 2008, the FAA revised the amount of data recorded for each incident, making the database 

even less informative. Some of the data fields are now blank for the most recent incidents. One 

of those fields was previously used to determine which of the Part 135 flights were scheduled 

and which were non-scheduled. As a result, it is not possible to present incident data separately 

for scheduled versus non-scheduled Part 135 operations among incidents later than 2007. For 

most of this report, the Part 135 incident data are restricted to 1996-2007 and all data from 2008-

2010 are ignored. 

In order to describe the types of aircraft which were involved in these accidents and incidents, 

the specific aircraft make and model (and in many cases, aircraft series) was determined for each 

accident and nearly all incidents. For the vast majority of events, this information could be easily 

found in the data record. For some events it was necessary to consult the FAA’s aircraft registry 

database. 

All aircraft in the data system for the chosen time period (1996-2010) were divided into groups 

based on some combination of engine type, aircraft use, aircraft size, and aircraft complexity. 

The aircraft categories are as follows: 

 
6 

x Jet engine 

o Wide Body Jet Airliners 

o Narrow Body Jet Airliners 

o Regional Jets 

o Medium Sized Business Jets 

o Small Business Jets (maximum takeoff weight <= 12,500 lbs) 

x Turbo-prop engine 

o Large Turbo-props (maximum takeoff weight >= 32,000 lbs and more than 30 seats) 

o Medium Turbo-props (12,500< maximum takeoff weight <32,000 lbs or 15-30 seats) 

o Small Turbo-props (maximum takeoff weight <12,500 lbs and less than 15 seats) 

x Reciprocating engine 

o Heavier multiple reciprocating engines (maximum takeoff weight >15,000 lbs) 

o Lighter multiple reciprocating engines (maximum takeoff weight < 15,000 lbs) 

o Single reciprocating engine, retractable landing gear 

o Single reciprocating engine, fixed landing gear 

o Light Sport Aircraft 

The systems involved in the failures and malfunctions are presented separately in Section 2.1.1 

for four categories of flight operations (Part 121, Scheduled Part 135, Non-Scheduled Part 135, 

and Part 91). A separate analysis looks at SCFM in different categories of aircraft type in Section 

2.1.2. 

2.1.1. System/Component Failures/Malfunctions by Operation Category 

This section examines system/component failure/malfunction (SCFM) by flight operations. A 

summary of the SCFM events can be found in Table 2. The incident data for Part 135 are from 

1996-2007 only. Across all operation categories, between 15 and 22 percent of accidents (row 

“Accidents with SCFM”) during 1996-2010 involved a failure or malfunction of some aircraft 

system or component. The lowest proportion of accidents and fatal accidents associated with 

SCFM was in Part 91 (12-15%, column “Part 91”), while the lowest percentage of fatalities was 

in Scheduled Part 135 (9%). Among Part 121 flights, SCFM accounted for 16 percent of all 

accidents, 36 percent of fatal accidents, and 66 percent of all fatalities. In Part 135 flights, SCFM 

accounted for 19 to 22 percent of all accidents, 18 to 19 percent of fatal accidents, and 9 to 19 

percent of all fatalities. Between 36 and 62 percent of all incidents included SCFM across all 

operation categories; the lowest percentage again was within Part 91. 

 
7 

Table 2. Summary of System/Component Failure/Malfunction Accidents and Incidents 

by Operation Category 

For each accident and incident, the system affected by the malfunction or failure was determined 

(see Table 3). In some events multiple systems were affected, and in these cases the first system 

affected was selected. For example, if an electrical malfunction preceded an engine fire, that 

event was categorized under “Electrical”. Engine and landing gear failures/malfunctions 

combined (highlighted rows) for between 56 and 69 percent of all SCFM accidents, and between 

48 and 79 percent of all SCFM incidents. No other single system accounted for more than fifteen 

percent of the failure/malfunction accidents or incidents. 

 

Type of Event Part 121 Part 135—

Scheduled 

Part 135—

Non-Scheduled Part 91

Total Accidents 619 108 768 17,628 

Accidents with SCFM 97 (15.7%) 20 (18.5%) 167 (21.7%) 2,581 (14.6%) 

Fatal Accidents 36 16 173 3,260 

Fatal SCFM 

Accidents out of all 

Fatal Accidents 

13 (36.1%) 3 (18.8%) 31 (17.9%) 387 (11.9%) 

Total Fatalities 1,190 94 427 6,295 

Fatalities in accidents 

with SCFM 782 (65.7%) 8 (8.5%) 81 (19.0%) 811 (12.9%) 

Total Incidents 4,890 295 1,343 17,411 

Incidents with SCFM 3008 (61.5%) 180 (61.0%) 759 (56.5%) 6323 (36.3%) 
8 

Table 3. Initial System Affected by Failure or Malfunction in Accidents and Incidents, 

 by Operation Category (with tall poles highlighted) 

For the purpose of further examination, the systems were divided into four groups (engine or fuel 

system, flight controls or structure, landing gear or hydraulics, and everything else). In the 

majority of accidents and incidents involving flight control failures or malfunctions, the issue 

was with the control surface itself, or with a cable leading to the control surface, rather than with 

the switch, selector or computer which controlled the movement of the surface; thus, it seemed 

reasonable to group these with other structural (mostly wing) failures. Similarly, most cases of 

System Part 121 Part 135—

Scheduled 

Part 135—

NonScheduled Part 91

Total SCFM Accidents 97 20 167 2581 

Electrical 8 (8.2%) 1 (5.0%) 7 (4.2%) 110 (4.3%) 

Engine 28 (28.9%) 7 (35.0%) 60 (35.9%) 1,133 (43.9%) 

Flight Controls 7 (7.2%) 1 (5.0%) 8 (4.8%) 144 (5.6%) 

Fuel 1 (1.0%) 1 (5.0%) 12 (7.2%) 226 (8.8%) 

Hydraulic 9 (9.3%) 1 (5.0%) 10 (6.0%) 63 (2.4%) 

Instrumentation/ 

Communication/Navigation 2 (2.1%) 1 (5.0%) 0 (0.0%) 28 (1.1%) 

Landing Gear 26 (26.8%) 6 (30.0%) 54 (32.3%) 635 (24.6%) 

Structure 6 (6.2%) 0 (0.0%) 9 (5.4%) 185 (7.2%) 

Other 10 (10.3%) 2 (10.0%) 5 (3.0%) 36 (1.4%) 

Unknown 0 (0.0%) 0 (0.0%) 2 (1.2%) 20 (0.8%) 

Total SCFM Incidents 3,008 180 759 6,323 

Electrical 133 (4.4%) 8 (4.4%) 36 (4.7%) 479 (7.6%) 

Engine 834 (27.7%) 48 (26.7%) 211 (27.8%) 1,729 (27.3%) 

Flight Controls 309 (10.3%) 11 (6.1%) 14 (1.8%) 124 (2.0%) 

Fuel 103 (3.4%) 6 (3.3%) 29 (3.8%) 249 (3.9%) 

Hydraulic 234 (7.8%) 11 (6.1%) 30 (4.0%) 160 (2.5%) 

Instrumentation/ 

Communication/Navigation 45 (1.5%) 2 (1.1%) 5 (0.7%) 44 (0.7%) 

Landing Gear 612 (20.3%) 64 (35.6%) 352 (46.4%) 3,288 (52.0%) 

Structure 128 (4.3%) 13 (7.2%) 37 (4.9%) 114 (1.8%) 

Other 610 (20.3%) 17 (9.4%) 45 (5.9%) 136 (2.2%) 
9 

hydraulic failure led to the inoperability of the landing gear or brakes, providing a natural link 

between those two systems. 

Table 4. Event Characteristics by Failure/Malfunction System Group and by Operation 

Category 

System Group Event 

Characteristics Part 121 Part 135—Scheduled 

Part 135— 

NonScheduled Part 91 

Engine or Fuel 

System

Total Accidents 29 8 72 1359 

Fatal Accidents 4 (13.8%) 1 (12.5%) 14 (19.4%) 180 

(13.2%) 

Total Fatalities 42 2 41 354 

Aircraft Destroyed 3 (10.3%) 1 (12.5%) 15 (20.8%) 213 

(15.7%) 

Flight Controls 

or Structure 

Total Accidents 13 1 17 329 

Fatal Accidents 5 (38.5%) 0 10 (58.8%) 163 

(49.5%) 

Total Fatalities 397 0 24 358 

Aircraft Destroyed 5 (38.5%) 0 8 (47.1%) 141 

(42.9%) 

Landing Gear or 

Hydraulic

Total Accidents 35 7 64 698 

Fatal Accidents 0 1 (14.3%) 1 (1.6%) 1 (0.1%) 

Total Fatalities 0 5 4 2 

Aircraft Destroyed 0 1 (14.3%) 2 (3.1%) 11 (1.6%) 

Instrumentation, 

Communication, 

Navigation, 

Electrical, 

Other, 

Unknown 

Total Accidents 20 4 14 195 

Fatal Accidents 4 (20.0%) 1 (25.0%) 6 (42.9%) 43 (22.1%) 

Total Fatalities 343 1 12 97 

Aircraft Destroyed 6 (30.0%) 1 (25.0%) 5 (35.7%) 62 (31.8%) 

Table 4 shows the number of accidents, the number of fatal accidents, the total number of 

fatalities, and the proportion of aircraft destruction in each group of the accidents. Flight 

control/structural failures/malfunctions accounted for less than fifteen percent of SCFM 

accidents (from Table 3) but were the most deadly of the four groups (between 39 and 59 percent 

of fatal accidents from Table 4), and in general the most likely to result in aircraft destruction 
10 

(between 39 and 47 percent of aircraft destroyed). Landing gear/hydraulic malfunctions were 

very rarely fatal, regardless of flight operation, and rarely resulted in aircraft destruction. 

Fatalities and aircraft destruction associated with engine/fuel system malfunctions were more 

likely than with landing gear/hydraulic failures, but less likely than either of the other two 

groups. 

Figures 2-5 compare the number of accidents for each group of initially affected systems across 

three time periods. Among Part 121 accidents (Figure 2), flight control/structural malfunctions 

occurred the least in every time period, but decreased less over time than the other categories. 

Figure 2. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Part 121. 

Among Scheduled Part 135 accidents (Figure 3), only one accident included a flight 

control/structural malfunction (a jammed control yoke), and the number of engine/fuel system 

failures/malfunctions did not decrease over time. Among Non-Scheduled Part 135 accidents 

(Figure 4), the number of engine/fuel system malfunctions decreased substantially over time, 

and the number of flight control/structural malfunctions decreased slightly. In Part 91 (Figure 5), 

the number of flight control/structural malfunctions, and also the number of landing 

gear/hydraulic malfunctions, has remained nearly constant, while the number of engine/fuel 

system malfunctions has declined. 

0

5

10

15

20

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
11 

Figure 3. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Scheduled Part 135. 

Figure 4. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Non-Scheduled Part 135. 

0

1

2

3

4

5

6

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

10

20

30

40

50

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
12 

Figure 5. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Part 91. 

Figures 6-9 compare the number of incidents in each of the four groups of initially affected 

systems across three time periods. Among Part 121 and Part 135 incidents (Figures 6, 7, and 8), 

malfunctions in all four groups have decreased over time. In Part 91 (Figure 9), the number of 

flight control/structural malfunctions has been consistently low, and the incidence of all other 

malfunctions and failures has declined. 

Figure 6. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Part 121. 

0

100

200

300

400

500

600

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

50

100

150

200

250

300

350

400

450

500

550

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
13 

Figure 7. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Scheduled Part 135. 

Figure 8. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Non-Scheduled Part 135. 

0

10

20

30

40

50

1996-2000 2001-2005 2006-2007

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

25

50

75

100

125

150

175

200

1996-2000 2001-2005 2006-2007

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
14 

Figure 9. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Part 91. 

2.1.2. System/Component Failures/Malfunctions by Aircraft Category 

This section examines SCFM within aircraft engine types, rather than flight operation. A full 

breakdown analysis of aircraft categories based on some combination of engine type, aircraft 

use, aircraft size, and aircraft complexity involved in the 1996-2010 accidents and incidents can 

be found in Ref. 3. The data were grouped in this section as follows: 

1. Jet Engine Aircraft 

2. Turbo-Prop Engine Aircraft 

3. Reciprocating Engine Aircraft 

o Multiple Reciprocating Engine 

o Single Reciprocating Engine 

Figure 10 shows accident volumes of engine types by operation category. Jet aircraft were 

involved in 81 percent of Part 121 accidents. In contrast, 87 percent of Part 91 accidents involved 

aircraft with a single reciprocating engine, with 9 percent of the aircraft having multiple 

reciprocating engines and the remaining 4 percent spread out over the other 2 engine types. 

Scheduled Part 135 accidents occurred on basically three engine types, none of them jets: turboprops (30%), multiple reciprocating engines (30%) and single reciprocating engines (41%). 

Non-Scheduled Part 135 accidents predominantly involved aircraft with reciprocating engines, 

either single (40%) or multiple (31%), although nearly eight percent of the accident aircraft were 

jets and 21 percent had turbo-prop engines. 

0

200

400

600

800

1000

1200

1400

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
15 

Figure 10. Summary of Engine Type in Accidents by Operation Category. 

A similar summary of the engine types involved in the incidents can be found in Figure 11. 

Seventy-three percent of Part 121 incidents occurred on flights with jet aircraft. In contrast, 

aircraft with reciprocating engine were involved in 91 percent of Part 91 incidents and 57 percent 

of Non-Scheduled Part 135 incidents. Twenty-three percent of Non-Scheduled Part 135 incidents 

occurred on flights with turbo-prop engines, and another 19 percent occurred in jet aircraft. Fiftyone percent of Scheduled Part 135 incidents occurred on turbo-props, and another 45 percent in 

aircraft with reciprocating engines. 

81%

8%

19%

30% 21%

30% 31%

9%

41% 40%

87%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Part 121 Part 135 - Scheduled Part 135 - NonScheduledPart 91

Engine Type Distribution by Operation Category - Accidents

Jet Engine Aircraft Turbo-Prop Engine Aircraft

Multiple Reciprocating Engines Single Reciprocating Engine
16 

Figure 11. Summary of Engine Type in Incidents by Operation Category. 

Table 5 summarizes the accident and incident data within the above engine types regardless of 

flight operation categories. Eighteen percent of jet and turbo-prop accidents (row “Accidents 

with SCFM”) included a system/component failure/malfunction. The highest proportions of fatal 

accidents and fatalities were in jets (20% in fatal accidents, and 54% in fatalities). In contrast, the 

lowest percentages of fatal accidents and fatalities were in single-engine aircraft (12% and 13%, 

respectively). 

Ninety percent of the reciprocating engine aircraft involved in accidents had a single engine 

(15,685/17,523); however, single engine accidents involving SCFM occurred less frequently 

(14%) than SCFM accidents with multiple engine aircraft (22%). 

Twenty-two percent of multiple-engine (reciprocating) aircraft accidents involved SCFM; this 

was the highest percentage among all engine types (row “Accidents with SCFM”). Interestingly, 

the lowest percentage was associated with single-engine aircraft (14 percent). Among incidents 

(row “Incidents with SCFM”), however, 58 to 61 percent of jet, turbo-prop, and multiple-engine 

(reciprocating) aircraft incidents involved SCFM, compared with 31 percent for single-engine 

incidents. 

73%

4%

19%

5%

25%

51% 23%

4%

30%

40%

17%

15% 17%

74%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Part 121 Part 135 - Scheduled Part 135 - NonScheduledPart 91

Engine Type Distribution by Operation Category - Incidents

Jet Engine Aircraft Turbo-Prop Engine Aircraft

Multiple Reciprocating Engines Single Reciprocating Engine

Could not be determined
17 

Table 5. Summary of System/Component Failure/Malfunction Accidents and Incidents by 

Engine type 

Table 6 shows the first system involved in a failure or malfunction for the accidents and 

incidents, broken down by engine type. Engine and landing gear failures/malfunctions combined 

for between 56 and 69 percent of all SCFM accidents, and between 49 and 84 percent of all 

SCFM incidents. No other single system accounted for more than fifteen percent of the 

failure/malfunction accidents or incidents. There were no fuel system malfunctions among 

accidents in jet aircraft during 1996-2010. 

 

Type of Event Jet Engines Turbo-Prop 

Engines 

Multiple 

Reciprocating 

Engines 

Single 

Reciprocating 

Engines 

All 

Reciprocating 

Engines 

Total Accidents 786 814 1,838 15,685 17,523 

Accidents with 

SCFM 

142 (18%) 149 (18%) 409 (22%) 2,165 (14%) 2,574 (15%) 

Fatal Accidents 85 240 590 2,570 3,160 

Fatal SCFM 

Accidents out of 

all Fatal 

Accidents 

17 (20%) 32 (13%) 88 (15%) 297 (12%) 385 (12%) 

Total Fatalities 1,416 767 1,267 4,556 5,823 

Fatalities in 

accidents with 

SCFM 

767 (54%) 126 (16%) 208 (16%) 581 (13%) 789 (14%) 

Total Incidents 4,722 2,437 3,624 13,270 16,894 

Incidents with 

SCFM 

2,741 

(58%) 

1,484 

(61%) 2,088 (58%) 4,073 (31%) 6,161 (36%) 
18 

Table 6. Initial System Affected by Failure or Malfunction in Accidents and Incidents, 

 by Engine type (with tall poles highlighted) 

 

System Jet 

Engines 

TurboProp 

Engines 

Multiple 

Reciprocating 

Engines 

Single 

Reciprocating 

Engines 

All 

Reciprocating 

Engines 

Total SCFM Accidents 142 149 409 2,165 2,574 

Electrical 12 (8%) 4 (3%) 16 (4%) 94 (4%) 110 (4%) 

Engine 26 (18%) 51 (34%) 113 (28%) 1,038 (48%) 1,151 (45%) 

Flight Controls 13 (9%) 10 (7%) 19 (5%) 118 (5%) 137 (5%) 

Fuel 0 (0%) 8 (5%) 28 (7%) 204 (9%) 232 (9%) 

Hydraulic 17 (12%) 8 (5%) 12 (3%) 46 (2%) 58 (2%) 

Instrumentation/ 

Communication/

Navigation 

3 (2%) 3 (2%) 8 (2%) 17 (1%) 25 (1%) 

Landing Gear 54 (38%) 46 (31%) 167 (41%) 454 (21%) 621 (24%) 

Structure 5 (4%) 13 (9%) 34 (8%) 148 (7%) 182 (7%) 

Other 11 (8%) 6 (4%) 7 (2%) 30 (1%) 37 (1%) 

Unknown 1 (1%) 0 (0%) 5 (1%) 16 (1%) 21 (1%) 

Total SCFM Incidents 2,741 1,484 2,088 4,073 6,161 

Electrical 122 (4%) 59 (4%) 109 (5%) 369 (9%) 478 (8%) 

Engine 703 

(26%) 

403 

(27%) 367 (18%) 1,369 (34%) 1,736 (28%) 

Flight Controls 

276 

(10%) 87 (6%) 29 (1%) 69 (2%) 98 (2%) 

Fuel 92 (3%) 46 (3%) 46 (2%) 204 (5%) 250 (4%) 

Hydraulic 205 (7%) 97 (7%) 78 (4%) 63 (2%) 141 (2%) 

Instrumentation/ 

Communication/

Navigation 

32 (1%) 28 (2%) 5 (0%) 30 (1%) 35 (1%) 

Landing Gear 633 

(23%) 

461 

(31%) 1,390 (67%) 1,893 (46%) 3,283 (53%) 

Structure 126 (5%) 100 (7%) 39 (2%) 35 (1%) 74 (1%) 

Other 552 

(20%) 

203 

(14%) 25 (1%) 41 (1%) 66 (1%) 
19 

As before, the systems were divided into four groups (engine or fuel system, flight controls or 

structure, landing gear or hydraulics, and everything else). Table 7 shows various characteristics 

of accidents in these four groups, by aircraft groups determined by the type of engine. Among jet 

aircraft, the most frequently affected systems are landing gear or hydraulic, whereas engine or 

fuel system malfunctions are by far most frequent among aircraft with reciprocating engines 

(row “Total Accidents”). In turbo-props, these two groups are similar. Regardless of engine type, 

flight controls or structural malfunctions are most likely, and landing gear or hydraulic failures 

least likely, to lead to fatal injuries and aircraft destruction (rows “Fatal Accidents” and “Aircraft 

Destroyed”). 

Table 7. Event Characteristics by Failure/Malfunction System Group and by Engine Type 

System Group Event 

Characteristics Jet Engines Turbo-Prop Engines 

Reciprocating 

Engines 

Engine or Fuel 

System

Total Accidents 26 59 1383 

Fatal Accidents 4 (15.4%) 10 (16.9%) 185 (13.4%) 

Total Fatalities 45 29 365 

Aircraft Destroyed 3 (11.5%) 15 (25.4%) 214 (15.5%) 

Flight Controls or 

Structure

Total Accidents 18 23 319 

Fatal Accidents 6 (33.3%) 14 (60.9%) 158 (49.5%) 

Total Fatalities 367 69 343 

Aircraft Destroyed 6 (33.3%) 13 (56.5%) 135 (42.3%) 

Landing Gear or 

Hydraulic

Total Accidents 71 54 679 

Fatal Accidents 1 (1.4%) 1 (1.9%) 1 (0.1%) 

Total Fatalities 4 5 2 

Aircraft Destroyed 1 (1.4%) 1 (1.9%) 12 (1.8%) 

Instrumentation, 

Communication, 

Navigation, 

Electrical, Other, 

Unknown 

Total Accidents 27 13 193 

Fatal Accidents 6 (22.2%) 7 (53.8%) 41 (21.2%) 

Total Fatalities 351 23 79 

Aircraft Destroyed 8 (29.6%) 5 (38.5%) 61 (31.6%) 

Figures 12-15 compare the number of accidents in these four system groups across three time 

periods by engine type. Figures 16-19 similarly compare the number of incidents. Among jet 

aircraft (Figure 12), the number of landing gear/hydraulic malfunctions decreased over time, but 

remained the most frequent type of malfunction. The other three system groups did not change 

substantially, but all were least frequent during 2001-2005. 
20 

Figure 12. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Jet Aircraft. 

Among turbo-prop aircraft (Figure 13), flight controls/structural malfunctions have increased, 

while engine/fuel and landing gear/hydraulic malfunctions have decreased. 

Figure 13. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Turbo-Prop Aircraft. 

Among multiple reciprocating engine aircraft (Figure 14), landing gear/hydraulic malfunctions 

have increased, while engine/fuel malfunctions have decreased. The number of flight 

controls/structural malfunctions has not changed much. 

0

5

10

15

20

25

30

35

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

5

10

15

20

25

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
21 

Figure 14. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Multiple Reciprocating Engine Aircraft. 

Among single reciprocating engine aircraft (Figure 15), engine/fuel malfunctions have decreased 

substantially, while all other types of malfunctions have decreased only slightly. 

Figure 15. Number of Accidents in Groups of Initially Affected Systems Across Three Time 

Periods for Single Reciprocating Engine Aircraft. 

 

0

10

20

30

40

50

60

70

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

50

100

150

200

250

300

350

400

450

500

550

1996-2000 2001-2005 2006-2010

Number of Accidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
22 

The number of incidents in all four system groups decreased over time for all engine types 

(Figures 16-19). The general trend was that within each time period, the descending order by 

frequency was landing gear/hydraulic, engine/fuel, other, and flight controls/structural. An 

exception to this general trend was for jet aircraft during 1996-2000, where engine/fuel 

malfunctions were the most frequent. 

Figure 16. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Jet Aircraft. 

Figure 17. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Turbo-Prop Aircraft. 

0

100

200

300

400

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

50

100

150

200

250

300

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
23 

Figure 18. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Twin Reciprocating Engine Aircraft. 

Figure 19. Number of Incidents in Groups of Initially Affected Systems Across Three Time 

Periods for Single Reciprocating Engine Aircraft. 

 

0

100

200

300

400

500

600

700

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems

0

100

200

300

400

500

600

700

800

1996-2000 2001-2005 2006-2010

Number of Incidents 

Engine or Fuel System Flight Control or Structure

Landing Gear or Hydraulic All Other Systems
24 

2.2. Analysis of ASRS Data 

AvSP systems analysis personnel have recently conducted a study of the NASA Aviation Safety 

Reporting System (ASRS) incident data. The incidents in the ASRS are reported voluntarily, and 

are subject to self-reporting biases. These incidents are not verified by the Federal Aviation 

Administration (FAA) or the National Transportation Safety Board (NTSB). Voluntary incident 

reports cannot be considered a representative sample of the underlying population of events they 

describe (Ref. 4). As such, this data cannot be used for statistical or trend analysis, but more for 

identifying vulnerabilities and gain a better understanding of the root causes of incidents and 

should be considered to compliment the data generated by mandatory, statistical, and monitoring 

systems. 

The aircraft related incidents with known SCFM system categories were analyzed. There were 

20,874 SCFM-related incidents (or reports) for Part 121, 135 and 91 operations during the time 

period from January 1993 through December 2012. Seventy-three percent of reported incidents 

were for Part 121, 5% for Part 135, and 22% for Part 91. Prior to March 1997, Part 121 

operations included aircraft with 30 or more seats. In March 1997, the definition of Part 121 

operations changed and now includes those aircraft with ten or more seats. 

The SCFM categories used in the analysis are the following, in alphabetical order: 

1. Automated Flight Controls 

2. Brakes 

3. Communication 

4. Control Surface 

5. Electrical / Power 

6. Environmental Control System 

7. Fuel System 

8. Furnishings and Equipment 

9. Hydraulics / Pneumatic 

10. Icing 

11. Landing Gear 

12. Miscellaneous 

13. Monitoring and Management 

14. Navigation 

15. Oil System 

16. Propulsion System 

17. Structures 

18. Weather System 

The aircraft related incidents were further identified in the ASRS dataset as having either critical 

or less severe aircraft equipment problems. When aircraft related incidents were caused by 

system component failure or malfunction, about 76% of the time they were identified as having a 

critical aircraft equipment problem. In addition to aircraft equipment problems, ASRS data 
25 

provided the event results of the aircraft related incidents. The incidents can result in 

maintenance action, in unrelated maintenance action, or contained no information. For years 

prior to 1999, the maintenance action was not recorded in ASRS. Therefore, there were only 

9,822 SCFM-related incident reports available. The incidents resulting in maintenance action 

were applicable to the “Maintain Vehicle Safety between Major Inspections (MVS)” Technical 

Challenge. As such, they were included in the analyses. 

Results 

Analysis of trends in the number of aircraft related incidents with known SCFM over the past 20 

years (January 1993 through December 2012) can be seen in Figure 20. Part 121 had fewer 

reported incidents during 1993-1997, got higher during 1998-2002, dropped slightly during 

2003-2007, and increased significantly during 2008-2012. Part 135 had a slight decreasing trend 

from 1993 to 2007, but a slight increase during the past 5 years. The slight decrease from 1993-

1997 to 1998-2002 may be due to the definition change in March 1997 making a shift from Part 

135 volume to Part 121 volume. Part 91 had a rather flat trend from 1993 to 2007, but had an 

increase during the past 5 years. 

Figure 20. Incidents related to SCFM over four time periods 

The number of incidents related to SCFM with an identified aircraft equipment problem over the 

time periods can be seen in Figure 21. Trends for critical equipment problem were similar to 

those without identified equipment problem (Figure 20). For less severe equipment problem 

(Figure 21b), Part 121 stayed rather flat from 1993 to 2007, and increased significantly during 

2008-2012. Part 135 had a similar trend as its critical equipment problem. Part 91 had a slightly 

decreasing trend from 1993 to 2007, and had a significant increase during the past 5 years. 

0

1000

2000

3000

4000

5000

6000

1993-1997 1998-2002 2003-2007 2008-2012

Incident Count

SCFM Related Incidents over Time

Part 121 Part 135 Part 91
26 

The number of incidents related to SCFM resulting in maintenance action over the time periods 

can be seen in Figure 22. Part 121 had fewer reported incidents during 2003-2007, and an 

increased trend during 2008-2012. Part 135 had a slightly decreasing trend from 1999 to 2002, 

and a flat trend from 2003 to 2012. Part 91 had a rather flat trend from 1999 to 2012. 

(a) Critical aircraft equipment problem 

(b) Less severe aircraft equipment problem 

Figure 21. Incidents related to SCFM with identified aircraft equipment problem over 

four time periods 

0

1000

2000

3000

4000

5000

1993-1997 1998-2002 2003-2007 2008-2012

Incident Count

SCFM Related Incidents with Critical Aircraft 

Equipment Problem over Time

Part 121 Part 135 Part 91

0

500

1000

1500

2000

1993-1997 1998-2002 2003-2007 2008-2012

Incident Count

SCFM Related Incidents with Less Severe Aircraft 

Equipment Problem over Time

Part 121 Part 135 Part 91
27 

Figure 22. Number of incidents resulting in maintenance action over three time periods 

Analysis of trends in the number of incidents broken down by SCFM category is assessed next. 

Table 8 compares SCFM categories with different identified aircraft equipment problems. 

Because each level of equipment problems had different numbers of incidents, percent incidents 

by SCFM categories were computed with respect to each equipment problem. Therefore, the 

total percent over all categories for any given equipment problem and any given FAR Part is 100 

percent. The SCFM tall poles (highlighted cells) for incidents identified as having critical aircraft 

equipment problems were the same as those for all incidents regardless of aircraft equipment 

problems (i.e., column “Critical + Less Severe”) across all FAR Parts. 

For incidents with critical aircraft equipment problems, Part 121 had propulsion system, control 

surface, and monitoring and management as the top three tall poles. Part 135 had propulsion 

system, landing gear, and monitoring and management as the top three tall poles. Part 91 had 

propulsion system, landing gear, and electrical or power as the top three tall poles. Across all 

FAR Parts, propulsion system was the tallest pole. 

For Part 121, when control surface and monitoring and management were the causes of 

incidents, they could be identified as either critical or less severe equipment problems. When 

navigation was the cause of incidents, it was always identified as less severe. 

For Part 135, when landing gear and monitoring and management were the causes of incidents, 

they could be identified as either critical or less severe equipment problems. When 

communication was the cause, it was always identified as less severe. 

For Part 91, when electrical or power was the cause, it could be identified as either critical or less 

severe equipment problems. When landing gear was the cause, it was always identified as a 

0

500

1000

1500

2000

2500

3000

1999-2002 2003-2007 2008-2012

Incident Count

Incidents Resulting in Maintenance Action

Part 121 Part 135 Part 91
28 

critical equipment problem. When communication, monitoring and management, and navigation 

were the causes, they were always identified as less severe. 

Table 8. Percent incidents by SCFM categories for different aircraft equipment problems

(with tall poles highlighted) 

SCFM categories

Aircraft equipment 

problem for Part 121 

Aircraft equipment 

problem for Part 135 

Aircraft equipment 

problem for Part 91 

Critical 

Less Severe 

Critical + 

Less Severe 

Critical 

Less Severe 

Critical + 

Less Severe 

Critical 

Less Severe 

Critical + 

Less Severe 

Automated Flight 

Controls 

3% 7% 4% 3% 5% 3% 3% 8% 4% 

Brakes 2% 2% 2% 4% 3% 3% 5% 3% 5% 

Communication 1% 5% 2% 2% 15% 5% 3% 25% 7% 

Control Surface 11% 11% 11% 5% 6% 5% 4% 3% 4% 

Electrical / Power 8% 5% 7% 8% 1% 6% 13% 11% 13% 

Environmental Control 

System 

10% 7% 9% 5% 6% 5% 4% 1% 3% 

Fuel System 3% 2% 3% 4% 2% 3% 5% 2% 5% 

Furnishings and 

equipment 

2% 4% 2% 1% 3% 1% 1% 1% 1% 

Hydraulics / Pneumatic 10% 5% 8% 3% 1% 3% 2% 1% 2% 

Icing 1% 1% 1% 1% 0% 1% 1% 0% 1% 

Landing Gear 7% 8% 7% 16% 12% 15% 18% 8% 16% 

Miscellaneous 1% 2% 2% 0% 8% 3% 1% 2% 1% 

Monitoring and 

management 

11% 13% 11% 14% 17% 14% 7% 11% 7% 

Navigation 4% 16% 8% 3% 11% 5% 3% 12% 5% 

Oil System 3% 1% 3% 3% 0% 2% 3% 1% 2% 

Propulsion System 17% 6% 14% 23% 6% 19% 26% 5% 22% 

Structures 4% 5% 4% 5% 6% 6% 4% 5% 4% 

Weather system 0% 1% 0% 0% 0% 0% 0% 0% 0% 

Table 9 compares percent incidents caused by SCFM breakdown, whether or not the incident 

results in maintenance action. The same top three SCFM tall poles for each FAR Part were 

observed regardless of the incidents results (i.e., with or without maintenance action). Part 121 

had propulsion system, control surface, and monitoring and management as the top three tall 

poles. Part 135 had propulsion system, landing gear, and monitoring and management as the top 

tall poles. Part 91 had propulsion system, landing gear, and electrical or power as the top three 

tall poles. Across all FAR Parts, propulsion system was the tallest pole.
29 

Table 9. Percent incidents by SCFM categories with incidents results (with top three tall 

poles highlighted) 

SCFM categories

Incident results for 

Part 121 

Incident results for 

Part 135 

Incident results for 

Part 91 

Maintenance 

action 

All results 

Maintenance 

action 

All results 

Maintenance 

action 

All results 

Automated Flight 

Controls 

3% 4% 2% 3% 4% 4% 

Brakes 2% 2% 5% 3% 6% 5% 

Communication 2% 2% 3% 5% 4% 7% 

Control Surface 13% 11% 6% 5% 4% 4% 

Electrical / Power 8% 7% 7% 6% 14% 13% 

Environmental Control 

System 

10% 9% 6% 5% 3% 3% 

Fuel System 3% 3% 4% 3% 5% 5% 

Furnishings and 

equipment 

2% 2% 2% 1% 1% 1% 

Hydraulics / Pneumatic 9% 8% 3% 3% 2% 2% 

Icing 1% 1% 1% 1% 1% 1% 

Landing Gear 8% 7% 18% 15% 18% 16% 

Miscellaneous 1% 2% 0% 3% 1% 1% 

Monitoring and 

management 

12% 11% 13% 14% 7% 7% 

Navigation 4% 8% 3% 5% 3% 5% 

Oil System 3% 3% 5% 2% 3% 2% 

Propulsion System 15% 14% 19% 19% 20% 22% 

Structures 5% 4% 4% 6% 5% 4% 

Weather system 0% 0% 0% 0% 0% 0% 

In addition to trends over time periods and trends by SCFM category, trends as an association 

between time periods and SCFM category are analyzed next. The percentages of incidents by 

SCFM category for all four time periods were summarized in Figures 23 and 24. Because each 

time period had different numbers of reported incidents, the percentages of incidents by 

categories were computed with respect to each period. As such, the total percentage over all 

SCFM categories for any given time period is 100 percent. 
30 

Figure 23. Incidents (%) by SCFM category with aircraft equipment problem over time (with tall poles highlighted)

1 4% 3% 3% 3% 10% 12% 5% 5% 6% 5% 3% 4% 1% 5% 4% 5% 2% 2% 4% 3% 3% 4% 5% 4%2 3% 2% 1% 2% 2% 2% 2% 2% 2% 2% 2% 2% 3% 3% 2% 5% 3% 6% 6% 5% 3% 5% 6% 5%3 1% 1% 1% 1% 10% 6% 3% 3% 4% 2% 2% 2% 5% 6% 6% 1% 6% 2% 3% 1% 15% 8% 5% 2%4 7% 11% 12% 12% 6% 9% 13% 12% 6% 11% 12% 12% 5% 2% 9% 6% 2% 3% 3% 5% 2% 2% 4% 5%5 7% 7% 8% 8% 4% 5% 6% 6% 6% 7% 8% 8% 6% 6% 6% 7% 14% 15% 14% 12% 13% 14% 13% 11%6 10% 11% 11% 10% 6% 8% 9% 6% 8% 11% 10% 9% 4% 5% 6% 8% 3% 4% 4% 4% 2% 3% 3% 4%7 3% 3% 3% 3% 2% 2% 3% 3% 3% 3% 3% 3% 2% 5% 2% 5% 5% 6% 5% 4% 4% 5% 5% 4%8 2% 2% 2% 2% 2% 7% 3% 3% 2% 3% 2% 2% 1% 1% 2% 2% 1% 0% 1% 1% 1% 0% 1% 1%9 6% 8% 9% 13% 2% 1% 6% 8% 4% 6% 8% 12% 3% 2% 1% 5% 0% 1% 2% 3% 0% 1% 2% 3%

10 1% 1% 1% 2% 0% 1% 1% 1% 1% 1% 1% 1% 0% 0% 1% 3% 1% 2% 1% 1% 1% 1% 0% 1%11 10% 7% 6% 7% 8% 5% 9% 10% 9% 6% 7% 8% 15% 17% 17% 12% 19% 18% 18% 16% 15% 16% 17% 16%12 1% 1% 2% 1% 3% 5% 2% 1% 2% 2% 2% 1% 7% 2% 0% 0% 1% 1% 1% 1% 3% 2% 1% 0%13 16% 13% 11% 8% 14% 10% 12% 15% 14% 12% 11% 10% 18% 17% 9% 8% 9% 7% 5% 6% 8% 7% 6% 8%14 4% 4% 5% 4% 17% 18% 12% 15% 11% 7% 7% 8% 5% 3% 5% 8% 2% 2% 3% 3% 4% 5% 5% 5%15 3% 3% 4% 3% 1% 0% 1% 1% 2% 3% 3% 3% 1% 2% 5% 3% 1% 3% 2% 3% 1% 2% 2% 3%16 18% 18% 17% 17% 7% 4% 7% 6% 13% 15% 14% 13% 19% 18% 19% 18% 28% 24% 25% 27% 22% 20% 22% 24%17 5% 5% 4% 4% 6% 4% 5% 5% 5% 4% 4% 4% 5% 7% 7% 4% 3% 4% 4% 4% 3% 4% 4% 4%18 0% 0% 0% 0% 0% 1% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0%

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

Aircraft equipment problem 

for Part 135 SCFM 

categories Critical Critical + Less Severe

Aircraft equipment problem for Part 121 Aircraft equipment problem for Part 91

Critical Less Severe Critical + Less Severe Critical + Less Severe

1 Automated Flight Controls 10 Icing2 Brakes 11 Landing Gear3 Communication 12 Miscellaneous4 Control Surface 13 Monitoring and management5 Electrical / Power 14 Navigation6 Environmental Control System 15 Oil System7 Fuel System 16 Propulsion System8 Furnishings and equipment 17 Structures9 Hydraulics / Pneumatic 18 Weather system
31 

Figure 24. Incidents (%) by SCFM category with event results over time (with tall poles highlighted)

Automated Flight Controls 4% 3% 3% 6% 5% 3% 4% 3% 4% 4% 3% 4% 5% 4%Brakes 2% 2% 2% 2% 2% 2% 2% 6% 6% 5% 3% 5% 6% 5%Communication 2% 2% 1% 4% 2% 2% 2% 6% 4% 2% 15% 8% 5% 2%Control Surface 12% 14% 12% 6% 11% 12% 12% 3% 4% 6% 2% 2% 4% 5%Electrical / Power 7% 9% 9% 6% 7% 8% 8% 17% 14% 13% 13% 14% 13% 11%Environmental Control System 12% 10% 7% 8% 11% 10% 9% 4% 3% 3% 2% 3% 3% 4%Fuel System 3% 3% 3% 3% 3% 3% 3% 4% 5% 6% 4% 5% 5% 4%Furnishings and equipment 2% 2% 2% 2% 3% 2% 2% 0% 1% 0% 1% 0% 1% 1%Hydraulics / Pneumatic 7% 8% 12% 4% 6% 8% 12% 1% 2% 2% 0% 1% 2% 3%Icing 1% 1% 1% 1% 1% 1% 1% 1% 0% 1% 1% 1% 0% 1%Landing Gear 6% 8% 9% 9% 6% 7% 8% 15% 19% 19% 15% 16% 17% 16%Miscellaneous 1% 2% 1% 2% 2% 2% 1% 1% 1% 1% 3% 2% 1% 0%Monitoring and management 12% 12% 11% 14% 12% 11% 10% 7% 5% 8% 8% 7% 6% 8%Navigation 4% 4% 4% 11% 7% 7% 8% 4% 2% 3% 4% 5% 5% 5%Oil System 3% 3% 3% 2% 3% 3% 3% 3% 3% 3% 1% 2% 2% 3%Propulsion System 16% 15% 13% 13% 15% 14% 13% 20% 21% 19% 22% 20% 22% 24%Structures 4% 4% 5% 5% 4% 4% 4% 5% 5% 5% 3% 4% 4% 4%Weather system 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0% 0%

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

1993-1997

1998-2002

2003-2007

2008-2012

SCFM categories Maintenance Action Incident results for Part 91 Maintenance Action All results All results Incident results for Part 121
32 

For Part 121, the tall poles changed slightly over the time periods for combined aircraft 

equipment problem (as shown by highlighted cells in column “Critical + Less Severe” of Figure 

23). Propulsion system (#16) and monitoring and management (#13) were two consistent tall 

poles over the past 20 years. Control surface (#4) became a tall pole in the past 15 years. 

Navigation (#14) was a tall pole in 1993-1997, but was not a tall pole in the past 15 years. 

Environmental control system (#6) was a tall pole from 1998 to 2007, but was not a tall pole in 

other periods. Hydraulics or pneumatic (#9) recently became a tall pole during 2008-2012. 

When breaking the incidents into the ones with critical aircraft equipment problem, Part 121 tall 

poles changed slightly over the time periods (as shown by highlighted cells in column “Critical” 

of Figure 23). Propulsion system (#16) and environmental control system (#6) were two of the 

tall poles from 1993 to 2012, with propulsion system being the tallest pole. Monitoring and 

management (#13) was one of the tall poles in 1993-2007, but was not a tall pole in the last 5 

years. Control surface (#4) became a tall pole in the past 15 years. Hydraulics/ pneumatic (#9) 

recently became a tall pole in the past 5 years. Landing gear (#11) was one of the tall poles from 

1993 to 1997, but was not a tall pole in the past 15 years. The significant increase in the number 

of incident reports during 2007-2012 (from Figure 21a) was due to a noticeably significant 

increase in the percentage of hydraulics/ pneumatic (#9) problems. 

For less severe equipment problem, the Part 121 top three tall poles changed slightly over the 

time periods (column “Less Severe” in Figure 23). Navigation (#14) and monitoring and 

management (#13) were two of the tall poles in 1993-2012. Control surface (#4) became a tall 

pole in the past 10 years. Automatic flight controls (#1) were a tall pole from 1993 to 2002, but 

were not a tall pole in the past 10 years. The significant increase in the number of incident 

reports during 2007-2012 (from Figure 21b) was not due to any specific SCFM category (no 

noticeable significant increase in percent by a particular category), but rather the tall poles were 

proportionally contributing to the increase in incidents. 

For Part 135, the tall poles have changed slightly over the four time periods. Due to a small 

number of incident reports across all SCFM categories for both aircraft equipment problems, 

only the combined equipment problem (“Critical + Less Severe”) was analyzed. Propulsion 

system (#16) and landing gear (#11) were two consistent tall poles over the past 20 years. 

Monitoring and management (#13) was a tall pole from 1993 to 2002, but was not a tall pole in 

the past 10 years. 

For Part 91, tall poles have not changed significantly over the time periods for the combined 

equipment problem. Propulsion system (#16), landing gear (#11), and electrical or power (#5) 

were consistently three of the tall poles over the past 20 years. Communication (#3) was a tall 

pole in 1993-1997, but was not a tall pole in the past 15 years. Due to a small number of 

incidents across all SCFM categories for less severe equipment problem, its trend of the SCFM 

tall poles was excluded in the analysis. For Part 91 with critical equipment problems, the tall 
33 

poles did not change in the past 20 years. Propulsion system (#16), landing gear (#11), and 

electrical or power (#5) were the top three tall poles for these time periods. 

Generally, trends of the SCFM tall poles over time for incidents having a critical equipment 

problem were similar to those for all incidents regardless of aircraft equipment problems for 

Parts 121 and 91. This may be because 76% of all incidents were the ones with a critical 

equipment problem. 

Figure 24 shows the breakdown of incidents by SCFM category considering the results of the 

incidents (with or without maintenance action). Trends of the SCFM tall poles over time for all 

FAR Parts for all incident results (i.e., with or without maintenance action) were the same as 

those in column “Critical + Less Severe” equipment problem of Figure 23. However, they were 

reported in this figure to facilitate the comparison. Due to the small number of incidents, Part 

135 was excluded. Clearly, trends of the SCFM tall poles over time for incidents resulting in 

maintenance action were the same as those for all results for Parts 121 and 91. 

For Part 121, the tall poles had changed slightly over the three time periods for incidents 

resulting in maintenance action. Propulsion system, control surface, and monitoring and 

management constantly were the top three tall poles from 1999 to 2012. Hydraulics or pneumatic 

recently became a tall pole during 2008-2012. Environmental control system was a tall pole from 

1999 to 2007, but was not a tall pole from 2008 to 2012. For Part 91 (Figure 24), the tall poles 

did not change from 1999 to 2012. Propulsion system, landing gear, and electrical or power were 

the top three tall poles across all three time periods. 

3. REVIEW OF VEHICLE HEALTH MANAGEMENT (VHM) ISSUES AND FUTURE 

RESEARCH NEEDS 

This section contains a review of subject matter experts’ safety priority lists and research studies 

pertaining to VHM issues. The study specifically focuses on VHM issues related to the MVS 

Technical Challenge (TC2). 

3.1. CAST Safety Enhancements Reserved for Future Implementation (SERFIs) 

The Commercial Aviation Safety Team (CAST) was established in 1998 in response to several 

commercial aviation accidents in the late 1990s (Ref. 5). It applies an integrated, data-driven 

strategy to reduce commercial aviation fatality risk in the United States and promote new 

government and industry safety initiatives throughout the world. 

Under the direction of a government and an industry co-chair, CAST sets overall policy and 

oversees the activities of the following working groups: Joint Safety Analysis Teams (JSATs) 

and Joint Safety Implementation Teams (JSITs). JSATs perform in-depth data analyses of a 

particular accident category, and then identify intervention strategies to eliminate potential 
34 

precursors and contributing factors to the accidents. JSITs develop safety enhancements based on 

the intervention strategies identified by the JSATs (Ref. 6). 

In addition to the current Safety Enhancements (SEs) in the CAST plan, there are 54 Safety 

Enhancements Reserved for Future Implementation (SERFIs), Research SERFIs, and Research 

& Development Safety Enhancements (R&D SEs) that were developed by various JSIT activities 

but were not approved by CAST for implementation on the active Safety Plan. 

An assessment of the CAST SERFIs against the MVS research products was conducted. The 

result of this assessment determined that MVS products are aligned with only one SERFI: R&D 

Safety Enhancement 118, Health and Usage Monitoring Systems (HUMS). HUMS is a R&D 

project focused on preventing Approach and Landing Accident Reduction (ALAR) accidents and 

incidents in commercial aviation (Refs. 7, 8, 9). While it contains high ratings of overall 

effectiveness and feasibility, HUMS was considered by the JSIT to require additional research 

before realizing the full potential to reduce accidents. As such, it was recommended to CAST as 

a research project, and it became a Safety Enhancement Reserved for Future Implementation 

(SERFI). 

Because SE 118 was recommended for research, a Detailed Implementation Plan delineating 

specific actions and outputs, identifying responsible organizations, and estimating financial 

resources required for completion was never developed. The JSIT merely developed the 

following Statement of Work outline for the HUMS project: 

Conduct research and develop technology for: 

• Detection, prediction and/or annunciation of impending equipment failures. 

• Detection and annunciation of inappropriate settings that may affect safe flight. 

• Real time decision making support for maintenance and operations. 

• Smart alerting systems that provide real time assistance to flight crews with on-board 

system failures and include diagnostics, prioritization schemes and elimination of nuisance 

alerts. 

MVS products applicable to SE 118 are: Hybrid Structural Damage Diagnosis (MVS-1.1), 

Vehicle Integrated Propulsion Research (MVS-2.1), Mitigating Turbomachinery Structural 

Failure (MVS-2.2), Vehicle Level Diagnostics and Integration (MVS-3.1), and Physics-Based 

Models and Algorithms for Wiring Fault Detection (MVS-3.2). 

 
35 

3.2. NTSB Most-Wanted List 

Every year the NTSB publishes a most-wanted list of transportation safety priorities for various 

transportation modes (e.g., aviation, highway, etc.) Although most of the NASA AvSP research 

is directed toward commercial aircraft operations, a portion of the work in MVS may be 

applicable to the issue of “Improve General Aviation Safety”, which is currently on the NTSB’s 

Most Wanted List (Ref. 10). A specific recommendation that may relate to MVS research is: 

“Aircraft maintenance workers should also be required to undergo recurrent training to keep 

them up to date with the best practices for inspecting and maintaining electrical systems, circuit 

breakers, and aged wiring.” 

In addition, the NTSB also issues safety recommendations as a result of accident investigations 

and other safety concerns that arise. Some recent open recommendations (during 2008-2013) that 

may be related to MVS research are listed in Table 10. 

Table 10. Recent NTSB Safety Recommendations related to MVS research 

Recommendation # Recommendation 

A-13-001 Establish duty-time regulations for maintenance personnel working under 

14 Code of Federal Regulations Parts 121, 135, 145, and 91 Subpart K 

that take into consideration factors such as start time, workload, shift 

changes, circadian rhythms, adequate rest time, and other factors shown 

by recent research, scientific evidence, and current industry experience to 

affect maintenance crew alertness. 

A-13-002 Encourage operators and manufacturers to develop and implement best 

practices for conducting maintenance under 14 Code of Federal 

Regulations Parts 135 and 91 Subpart K, including, but not limited to, the 

use of work cards for maintenance tasks, especially those involving 

safety-critical functions, that promote the recording and verification of 

delineated steps in the task that, if improperly completed, could lead to a 

loss of control. 

A-13-003 Require that personnel performing maintenance or inspections under 14 

Code of Federal Regulations Parts 121, 135, 145, and 91 Subpart K 

receive initial and recurrent training on human factors affecting 

maintenance that includes a review of the causes of human error, 

including fatigue, its effects on performance, and actions individuals can 

take to prevent the development of fatigue. 

A-12-068 Develop fire detection system performance requirements for the early 

detection of fires originating within cargo containers and pallets and, once 

developed, implement the new requirements. 

A-10-096 Require that mechanics performing required inspection item and other 

critical tasks receive on-the-job training or supervision when completing 

the maintenance task until the mechanic demonstrates proficiency in the 

task.
36 

3.3. NRC Decadal Survey of Civil Aeronautics 

The National Research Council (NRC) is a part of the National Academy of Sciences with the 

purpose of advising the government on matters of science and technology. It was organized in 

1916 and has become the principal operating agency of both the National Academy of Sciences 

and the National Academy of Engineering. The NRC conducted a decadal survey of civil 

aeronautics to assist NASA in developing a decadal strategy for federal aeronautics research in 

2006 (Ref. 11). The report describes research that is deemed necessary to further the state of the 

art in areas consistent with NASA’s legislative charter resulting in significant long term impact 

on national aeronautics. The survey study prioritized the identified research & technology 

(R&T) challenges in terms of supporting infrastructure, mission alignment, lack of alternative 

sponsors, and appropriate risk level according to NASA’s civil aeronautics research program. 

The NRC examined a total of 89 distinct R&T challenges that were categorized into five R&T 

areas. The five areas were 

A: Aerodynamics and aero acoustics. 

B: Propulsion and power. 

C: Materials and structures. 

D: Dynamics, navigation, and control, and avionics. 

E: Intelligent and autonomous systems, operations and decision making, human 

integrated systems, and networking and communications. 

Assessment of R&T challenges against the MVS research products was conducted. Table 11 

presents only the R&T challenges applicable to the MVS. For each challenge, numeric value 

following the letter of the R&T area indicates the NASA priority. The smaller the numeric, the 

higher the priority the challenge had. 

Table 11. R&T Challenges applicable to the MVS TC 

Challenge R&T Area Description 

B3 Propulsion and power Intelligent engines and mechanical power systems 

capable of self-diagnosis and reconfiguration 

between shop visits 

C1 Materials and structures Integrated vehicle health management 

D5 Dynamics, navigation, and 

control, and avionics 

Fault tolerant and integrated vehicle health 

management systems 

The committee also identified two key barriers to achieving the aeronautics objectives: (1) 

certification and (2) change management. As systems become more complex and 

nondeterministic, methods to certify new technologies become more difficult to validate. It is 

essential for NASA, with collaboration with the FAA, to conduct research on certification 

methods and on methods to introduce the anticipated change. 
37 

3.4. Future Aviation Safety Team Areas of Change 

The European Commercial Aviation Safety Team (ECAST) and the Commercial Aviation Safety 

Team (CAST) in the USA sponsor the Future Aviation Safety Team (FAST) (Ref. 12). FAST 

performs analyses of ongoing and future changes affecting the global aviation system with the 

goal of revealing unidentified safety hazards. Beginning in 2001, FAST compiled a database of 

Areas of Change (AoC). An AoC is defined as “any phenomenon that will affect the safety of the 

aviation system either from within or from important domains external to aviation” (Ref. 13). 

The current database that contains AoC was recently updated on November 15, 2013. Potential 

safety hazards associated with each AoC are also available in the database. Table 12 presents 

AoCs that are related to the MVS Technical Challenge. Some of the hazards cited include: 

x high complexity vehicle health management systems that are unable to yield to software 

certification techniques that exist today 

x failure to detect aircraft structural damage due to particle shedding, de-lamination, and 

high-current lightning strikes 

x certification challenges due to non-deterministic nature of Artificial Intelligence outputs 

from integrated modular architecture 

x the shortage of certified maintenance personnel 

x inadequate maintenance skills and trainings 

x crew’s fatigue causing maintenance errors. 
38 

Table 12. Future Aviation Safety Team Areas of Change applicable to the MVS TC (Ref. 13) 

AoC Number - Title Description Potential Hazards 14 - Advanced vehicle health management systems 

Future vehicle health systems may be based on continuously updated vehicle state matrices derived from networks of multiple sensors. Advanced software models incorporating the functional characteristics of the vehicle may process the sensor network outputs.

x Systems of such complexity that they are unable 

to yield to software certification techniques that exist today. In some cases it is not the software itself that is the issue; it is the failed logic that 

drives annunciations and/or changes especially 

following system degradation/failures. 

x Sensors continuing to be the lowest reliability 

components and therefore need to be redundant to obtain the required system safety 

x Sensor failures producing single point failure of 

multiple devices 

39 – Increasing use of composite structural materials

The use of composites will continue to increase in aircraft structures.

x Failure to detect sub-surface damage and delamination x Shedding of micron-sized particles due to fatigue 

and chafing 

x Damage due to high-current lightning strikes 
39 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 185 – Introduction of Non-Deterministic 

Approaches (NDA) and artificial intelligence (AI; i.e. self-learning) in 

aviation systems

Complex engineered products are more likely to meet performance requirements when NDA are used. Aircraft structural health management has always relied upon NDA. Management of the Next Generation Air Transportation System (NextGen) will use NDA for trajectory-based operations (TBO) 

to account for aircraft position and weather uncertainty. Future flight decks may contain, or be expected to interact with, software “intelligent agents.” The characteristics of these agents may 

differ significantly from most software tools in use today. The increasing complexity of technology drives the need for such NDA. 

x Certification challenges due to non-deterministic 

nature of AI outputs from integrated modular architectures 

x Pilots not understanding intent and actions of AI 

avionics 

x Failure to achieve robustness, as defined under 

DO-178C1 guidelines – the very specific proof 

that under all application failure conditions, a 

single failure in one partition will not affect other partitions. 

1 DO-178C is Software Considerations in Airborne Systems and Equipment Certification. This replaces DO-178B as the primary document by which the 

certification authorities such as FAA, EASA and Transport Canada will approve all commercial software-based aerospace systems. The new document is called 

DO-178C/ED-12C and was completed in November 2011 and approved by the RTCA, Inc. in December 2011. It became available for sale and use in January 2012. RTCA, Inc. (known as Radio Technical Commission for Aeronautics, originally founded in 1935, and until its re-incorporation in 1991 as a private notfor-profit corporation) is a US volunteer organization that develops technical guidance for use by government regulatory authorities and by industry. It has over 200 committees and overall acts as an advisory body to the FAA. 
40 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 226 – Changes in the qualifications of maintenance personnel

The shortage of certified maintenance personnel may result in lower quality servicing and maintenance of aircraft with a concomitant reduction in the reliability of both new and aging aircraft. Servicing of advanced avionics will require specialized skills, yet training in disciplines such as composite material repair, nondestructive inspection, solid-state electronics/avionics/built-in test equipment, principles of troubleshooting and human factor is currently only an option within maintenance training curricula. As the number of non-certified staff increases, the need to check their work increases. 

x Acceptance of poor quality work either because 

of time limitations or because errors are not detected. 

x Reduction in the availability of certified 

maintenance personnel due to tightening of controls on maintenance procedures, limitation of working hours, vision tests, etc. 

x Reduction in the number of experienced 

maintenance inspectors. 
41 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 230 - Paradigm shift from paper based to electronic based maintenance records and databases

In the future, complex, integrated aircraft will require more and more automation for fault detection, diagnosis, and resolution. In addition, new diagnostic and prognostic safety analysis will require electronic tracking of maintenance findings and actions.

x Degradation in maintenance quality of legacy 

aircraft which were previously paper-based but are transitioning to a computerized format 

x Inappropriate skill sets among maintenance 

personnel because of changing processes, tools, and techniques to support the new computerized systems 

x Poor task verification processes x Lack of coordination between maintenance and 

flight crews 

x Disconnect in processes for handling the formal 

aircraft log; manual, via automation 

x Failure of processes to fully inform crew of 

inadequate pre-flight aircraft status due to new 

electronic log entry formats; mismatches between 

manual, paper logs and electronic logs 

x As with any digital system, it is not enough to 

make “digital” copies of paper (scanning, PDFs). It is critical to build in “smart” tags, indexes and 

cross-references so information can be navigated and “found.” 

x Loss of access to existing maintenance 

information during transition process to electronic records. 

x Cumbersome access to historic maintenance 

records required to be kept by aircraft owner. 
42 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 236 - Increasing use of virtual mockups for maintenance training and for evaluation of requirements

Digital/electronic mock-ups are now being adopted by the industry as substitutes for the physical mockups. It should be recognized that the current digital mock-up capability together with available human 

modeling capability does not permit total 

maintenance/assembly task simulation (perhaps 2-5 years away). While any safety related risk is low, if a situation is not recognized during design phase, it will not emerge or be addressed until assembly of first aircraft. This results in a cost/schedule penalty 

and aircraft maintainability issues.

x Maintenance errors arising from differences 

between the training environment and real line operations. 

x Failure to maintain configuration control between 

maintenance simulators and actual aircraft physical hardware. 

This should not be a hazard if:

x The use of various levels of simulation are 

carefully assessed and used accordingly. 

x Assessment should include testing that ensures 

there is no negative transfer of learning from simulator to operations. 
43 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 241 - Operational tempo and economic considerations affecting fatigue among maintenance personnel

As result of increased financial pressure on airlines over the last 10-15 years there have been changes in 

the way maintenance organizations conduct their work. The number of maintenance employees per aircraft has been reduced significantly even taking into consideration that the present fleet demands less maintenance due to increased quality and more 

efficient maintenance programs. Only in specific maintenance tasks such as primary flight control work this ratio is still more or less normal. For almost all other tasks there are now just spots checks of 10-15% of the work actually performed by someone else. Contract maintenance personnel have economic incentives to seek out overtime to maximize their income. A large number of countries still have not set maximum duration working times for maintenance staff like there are for pilots. Due to tight daytime flight schedules, there is increasing pressure for nightshift operations (there is a known safety risk when working under pressure in night hours on complicated work) on the involved maintenance organization. 

x Reduction in staff, economic incentives available 

to maintenance technicians plus shifts toward night schedules for critical maintenance increase the likelihood of fatigue and maintenance errors. Due to tight daytime flight schedules, there is increasing pressure for nightshift operations on the involved maintenance organization. 

x Many countries still have not set maximum 

duration working times for maintenance staff like there are for pilots. 

x Number of maintenance employees per aircraft 

has been reduced significantly. Only in specific 

maintenance tasks such as primary flight control 

work is this ratio is still more or less normal. 

x Based on aircraft sales forecasts in non-Western 

markets, there will be a worldwide shortage of 

qualified maintenance personnel. 

x The loss of experience, safety culture, and tribal 

knowledge may be a bigger issue than overwork 

and fatigue. 
44 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 251 – Introduction of new training methodologies for maintenance staff 

IATA2 Training and Qualification Initiative (ITQI) 

for Maintenance is a recent IATA initiative. It is centered on competency based training and assessment. This will require the definition of competencies. An approach has been validated through meetings with original equipment 

manufacturers, airline maintenance and training organizations. IATA has worked closely with ICAO to develop this material. The Air Navigation Council (ANC) was briefed in January 2010 on the progress. 

Lack of ICAO guidance material on how competency based training can be applied to maintenance.

2 International Air Transport Association (IATA) is an international trade association for the world’s airlines (240 airlines or 84% of total air traffic) (ref. 22). 

IATA is actively promoting flight safety in safety auditing, infrastructure safety, safety data management and analysis, integrated airline management system, flying operations, and cargo and dangerous goods safety. 
45 

Table 12. (Continued) 

AoC Number - Title Description Potential Hazards 256 - Decreasing availability of qualified maintenance staff at stations other than home base of operation

Increasing financial pressure on airlines is resulting in steady reductions of maintenance staff at out stations. As a result of this phenomenon, flight crew is increasingly reluctant to report aircraft defects when away from home base. Statistics bear this out. A 2009 Airline Engineers International survey revealed that airlines, including majors reported inbound defects as high as 94% over only 6% outbound defects. Until recently, such behavior may have resulted only in incidents, albeit that well documented cases are scarce. This phenomenon may 

have been a root cause of at least one recent accident (Turkish Airlines, Schiphol, 2009).

One of the largest contract labor providers is in the 

process of recruiting labor from outside the United States. This organization is currently in the process of 

gaining Department of Labor and Immigration and Naturalization approval in order to offer jobs and obtain appropriate visas. One of the potential sources being investigated is the UK, where they are also experiencing a shortage of qualified aircraft 

maintenance technicians. 

x Lack of timely servicing of aircraft with 

potentially flight-critical component or system problems. 

x Poor quality aircraft servicing due to hiring of 

minimally-qualified staff 

x Over-reliance on Minimum Equipment List 

(MEL) procedures as safety nets 

x Incorrect information on the MEL within the 

airline operation center 

x Inappropriate release of an aircraft by dispatch. 
46 

3.5. Naval Air Systems Command (NAVAIR) 

Naval Air Systems Command (NAVAIR) of the United State Navy solicited for research 

proposals in four research areas during Fiscal Year 2013 (Ref. 14). Two out of four research 

areas were likely to be related to MVS: BAA 121 – Advanced Aircraft Power Systems (for 

research up to four years); and BAA 124 – Propulsion and Power System, Condition Based 

Maintenance, Prognostics, Diagnostics, and Health Monitoring (for research up to five years). 

Under BAA 121, one of Navy power system needs was “diagnostics, prognostics, and health 

management for electrical power and wiring systems to bring about improvements in readiness, 

maintenance, safety, and cost” (Ref. 14). 

Under BAA 124, Navy seeks to “develop propulsion and power system condition based 

maintenance, prognostic, diagnostic, and health monitoring technologies to improve aircraft 

safety, reliability, maintainability, affordability, and availability” (Ref. 14). 

3.6. Industry 

Several aerospace corporations are actively developing technology related to the MVS research: 

General Electric (GE) Sensing and Inspection Technologies (Ref. 15); Airplane Health 

Management services at Boeing (Refs. 16, 17); Aircraft Maintenance Analysis (AIRMAN) at 

Airbus (Refs. 18, 19, 20); and Health and Usage Management Systems (HUMS) at UTC 

Aerospace Systems (Ref. 21). 

 
47 

4. DISCUSSION AND CONCLUSIONS 

NTSB accident data (1996-2010) and FAA incident data (1996-2010) were examined across four 

operational categories: Federal Aviation Regulations Part 121, Scheduled Part 135, NonScheduled Part 135, and Part 91. In addition, an analysis of the types of aircraft involved in the 

accidents and incidents was performed. The aircraft were divided into groups according to 

engine type. The system /component failure/malfunction (SCFM) data were examined both by 

operation category and by aircraft engine type. 

In Part 121 operations, jet aircraft were involved in 81 percent of the accidents and 73 percent of 

the incidents. In contrast, Part 91 and Non-Scheduled Part 135 accidents and incidents occurred 

predominantly on aircraft with reciprocating engines. A large number of Scheduled Part 135 

accidents (30 percent) and incidents (51 percent) occurred on turboprop aircraft. More than fifty 

percent of the jet fatalities occurred in accidents that included SCFM, compared with only 

thirteen percent of the fatalities in single-engine aircraft. 

Twenty-two percent of multiple-engine (reciprocating) aircraft accidents involved SCFM; this 

was the highest percentage among all engine types. Interestingly, the lowest percentage was 

associated with single-engine (reciprocating) aircraft (14 percent). Among incidents, however, 

58 to 61 percent of jet, turbo-prop, and multiple-engine (reciprocating) aircraft incidents 

involved SCFM, compared with 31 percent for single-engine incidents. 

Across all flight categories (i.e., FAR Parts 121, 135, and 91), the most common systems 

involved in SCFM accidents and incidents were the engine and landing gear. In general, failures 

or malfunctions of either the landing gear or hydraulic systems result in less severe outcomes 

(fewer fatalities and less aircraft destruction) than other systems, whereas failures or 

malfunctions of either the structure or flight control systems result in more severe outcomes. 

Similar observations were found when accidents and incidents were examined by aircraft engine 

types. 

The SCFM related incidents analysis of the ASRS dataset resulted in 20,874 reports during 

1993-2012. Seventy-three percent of reported incidents were for Part 121, 22% for Part 91, and 

5% for Part 135. The SCFM incidents were further identified in the ASRS dataset as having 

critical or less severe aircraft equipment problems. When aircraft related incidents were caused 

by system component failure or malfunction, about 76% of the time they were identified as 

having a critical aircraft equipment problem. In addition to aircraft equipment problems, ASRS 

data provided the event results of the aircraft related incidents. The incidents can result in 

maintenance action, in unrelated maintenance action, or contained no information. The incidents 

resulting in maintenance action were applicable to the MVS Technical Challenge. As such, they 

were included in the analyses. 

For incidents with critical aircraft equipment problems, Part 121 had propulsion system, control 

surface, and monitoring and management as the top three SCFM categories (tall poles). Part 135 
48 

had propulsion system, landing gear, and monitoring and management as the top three tall poles. 

Part 91 had propulsion system, landing gear, and electrical or power as the top three tall poles. 

Across all FAR Parts, propulsion system was the tallest pole. The SCFM tall poles for all 

incidents regardless of aircraft equipment problems were the same as those for incidents 

identified as having a critical aircraft equipment problem across all FAR Parts. 

For Part 121, when control surface and monitoring and management were the causes of 

incidents, they could be identified as either critical or less severe equipment problems. When 

navigation was the cause of incidents, it was always identified as less severe. 

For Part 135, when landing gear and monitoring and management were the causes of incidents, 

they could be identified as either critical or less severe equipment problems. When 

communication was the cause, it was always identified as less severe. 

For Part 91, when electrical or power was the cause, it could be identified as either critical or less 

severe equipment problems. When landing gear was the cause, it was always identified as critical 

equipment problem. When communication, monitoring and management, and navigation were 

the causes, they were always identified as less severe. 

When comparing SCFM-related incidents, whether or not the incident results in maintenance 

action, the same top three SCFM tall poles for each FAR Part were observed. Part 121 had 

propulsion system, control surface, and monitoring and management as the top three tall poles. 

Part 135 had propulsion system, landing gear, and monitoring and management as the top three 

tall poles. Part 91 had propulsion system, landing gear, and electrical or power as the top three 

tall poles. Across all FAR Parts, propulsion system was the tallest pole. 

An assessment of the CAST SERFIs against the MVS research products was conducted. The 

result of the assessment determined that MVS products are aligned with only one SERFI: R&D 

Safety Enhancement 118, Health and Usage Monitoring Systems (HUMS). Five out of six MVS 

products are applicable to SE 118, which are: Hybrid Structural Damage Diagnosis (MVS-1.1), 

Vehicle Integrated Propulsion Research (MVS-2.1), Mitigating Turbomachinery Structural 

Failure (MVS-2.2), Vehicle Level Diagnostics and Integration (MVS-3.1), and Physics-Based 

Models and Algorithms for Wiring Fault Detection (MVS-3.2). 

The majority of aviation safety improvements on the 2012 NTSB Most-Wanted List as well as 

recent open safety recommendations from 2008 to 2013 related to the MVS Technical Challenge 

involve reduction of maintenance crew errors due to crew’s fatigue and inadequate training. 

The NRC Decadal Survey of Civil Aeronautics related to the MVS research products was 

assessed. The survey study prioritized the identified research & technology (R&T) challenges 

according to NASA’s civil aeronautics research program. The NRC examined a total of 89 

distinct R&T challenges. Three challenges applicable to the MVS Technical Challenge were: B3 

– Intelligent engines and mechanical power systems capable of self-diagnosis and 
49 

reconfiguration between shop visits; C1 – Integrated vehicle health management; and D5 – Fault 

tolerant and integrated vehicle health management systems. The committee also identified two 

key barriers to achieving the aeronautics objectives: (1) certification and (2) change 

management. As systems become more complex and nondeterministic, methods to certify new 

technologies become more difficult to validate. It is essential for NASA, with collaboration with 

the FAA, to conduct research on certification methods and on methods to introduce the 

anticipated change. 

According to the Future Aviation Safety Team, areas of changes related to the MVS Technical 

Challenge are either vehicle-related or maintenance crew-related. Vehicle-related areas of 

changes were: AoC-14 Advanced vehicle health management systems; AoC-39 Increasing use of 

composite structural materials; and AoC-185 Introduction of Non-Deterministic Approaches 

(NDA) and artificial intelligence (self-learning) in aviation systems. Potential safety hazards 

associated with these vehicle-related areas of changes were: highly complex vehicle health 

management systems that are unable to yield to software certification techniques that exist today; 

failure to detect aircraft structural damage due to particle shedding, de-lamination, and highcurrent lightning strikes; and certification challenges due to non-deterministic nature of Artificial 

Intelligence outputs from integrated modular architecture. Several maintenance crew-related 

areas of changes were driven by the shortage of certified maintenance personnel, inadequate 

maintenance skills and trainings, and crew’s fatigue causing maintenance errors. 

Future research direction in the MVS Technical Challenge is evidently strong as seen from Fiscal 

Year 2013 research solicitations from NAVAIR, and MVS-related technologies actively being 

developed by aviation industry leaders, including GE, Boeing, Airbus, and UTC Aerospace 

Systems. Given the highly complex vehicle heath management systems, modifications can be 

made in the future so that the VSST technical challenges address inadequate maintenance crew’s 

trainings and skills, and the certification methods of such systems as recommended by the 

NTSB, NRC, and FAST areas of change. 

 
50 

ACRONYMS 

AIDS = Accident/Incident Data System 

ALAR = Approach and Landing Accident Reduction 

ASIAS = Aviation Safety Information Analysis and Sharing 

ASRS = Aviation Safety Reporting System 

AvSP = Aviation Safety Program 

CAST = Commercial Aviation Safety Team 

FAA = Federal Aviation Administration 

FAR = Federal Aviation Regulations 

FAST = Future Aviation Safety Team 

HUMS = Health and Usage Monitoring Systems 

IATA = International Air Transport Association 

ICAO = International Civil Aviation Organization 

JSAT = Joint Safety Analysis Team 

JSIT = Joint Safety Implementation Team 

MVS = Maintain Vehicle Safety between Major Inspections 

NAVAIR = Naval Air Systems Command 

NRC = National Research Council 

NTSB = National Transportation Safety Board 

R&D SE = Research & Development Safety Enhancement 

SCFM = System Component Failures or Malfunctions 

SE = Safety Enhancement 

SERFI = Safety Enhancements Reserved for Future Implementation 

VHM = Vehicle Health Management 

VSST = Vehicle Systems Safety Technology Project 

ACKNOWLEDGEMENT 

We thank Dr. Sharon Monica Jones and Colleen A. Withrow for valuable comments. 

 
51 

REFERENCES 

1. Krasa, P., and Graves, S., “Vehicle Systems Safety Technology (VSST) Project Plan,” 

National Aeronautics and Space Administration, February 1, 2013. 

2. Gibson, J.E., Scherer, W.T., and Gibson, W.F., “How to Do Systems Analysis,” John Wiley 

& Sons, Inc., Hoboken, New Jersey, 2007. 

3. National Transportation Safety Board, “Government Information Locator Service (GILS): 

Aviation Accident Synopses”’ http://ntsb.gov/Info/gils/gilssyn.htm. 

4. ASRS: The Case for Confidential Incident Reporting Systems 2001, NASA ASRS 

Publication; 

http://asrs.arc.nasa.gov/docs/rs/60_Case_for_Confidential_Incident_Reporting.pdf, accessed 

February 4, 2014. 

5. Commercial Aviation Safety Team, “The Commercial Aviation Safety Team,” 2011. 

http://www.cast-safety.org/. 

6. Federal Aviation Administration, “Fact Sheet – Commercial Aviation Safety Team,” July 11, 

2013. http://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=15214. 

7. Joint Safety Analysis Team (JSAT), “Results and Analysis, Approach and Landing Accident 

Reduction (ALAR),” September 10, 1999. 

8. Joint Safety Implementation Team (JSIT), “Results and Analysis, Approach and Landing 

Accident Reduction (ALAR),” May 17, 2001. 

9. Commercial Aviation Safety Team, Approach and Landing Accident Reduction (ALAR) 

Joint Safety Implementation Team (JSIT), “Implementation Plan for Approach and Landing 

Health & Usage Monitoring Systems (HUMS),” October 1, 2002. 

10. National Transportation Safety Board, “NTSB Most Wanted List – Improve General 

Aviation Safety,” November 14, 2012. http://www.ntsb.gov/safety/mwl5_2012.html, 

Accessed December 2013. 

11. National Research Council, “Decadal Survey of Civil Aeronautics,” 2006. 

12. Future Aviation Safety Team, http://www.nlr-atsi.nl/fast/, Accessed February 12, 2014. 

13. Future Aviation Safety Team, “Areas of Change Catalog: Ongoing and Future Phenomena 

and Hazards Affecting Aviation”, http://www.nlr-atsi.nl/fast/FAST_AoCs_20131115.pdf, 

Accessed February 12, 2014. 

14. “Guide to Naval Aviation Propulsion and Power Technology Development Programs,” 

Naval Air Systems Command, Propulsion and Power Engineering, Patuxent River, MD, 

October 2012. 

15. “GE Sensing and Inspection Technologies,” http://www.ge-mcs.com/download/itcommon/GEIT-10012EN_ndt-brochure.pdf, Accessed November 2013. 

16. “The Boeing Edge -- Airplane Health Management,” 

http://www.boeing.com/boeing/commercial/aviationservices/information-services/real-timeoperations/ahm.page, Accessed November 2013. 
52 

17. “The Boeing Edge – Real-Time Operations,” 

http://www.boeing.com/boeing/commercial/aviationservices/information-services/real-timeoperations/index.page?, Accessed November 2013. 

18. “Airbus, The Leading Aircraft Manufacturer, AIRMAN real-time aircraft health monitoring”, 

http://www.airbus.com/innovation/proven-concepts/in-fleet-support/airman/, Accessed 

November 2013. 

19. “AIRTHM – Airbus Real Time Health Monitoring,” 

http://www.airbus.com/support/maintenance-engineering/airthm/, Accessed November 2013. 

20. “Airbus E-Solutions,” http://www.airbus.com/support/maintenance-engineering/e-solutions/, 

Accessed November 2013. 

21. “Sensors and Integrated Systems -- Health and Usage Management Systems (HUMS),” 

http://utcaerospacesystems.com/cap/products/Pages/health-usage-management-systems.aspx, 

Accessed November 2013. 

22. International Air Transport Association, http://www.iata.org/about/Pages/index.aspx, 

Accessed February 13, 2014. 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Identification of Vehicle Health Assurance Related Trends 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Phojanamongkolkij, Nipa; Evans, Joni K.; Barr, Lawrence; Leone, Karen; 

Reveley, Mary S.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20382

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

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Unclassified - Unlimited

Subject Category 66

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

Trend analysis in aviation as related to vehicle health management (VHM) was performed by reviewing the most current statistical and prognostics data 

available from the National Transportation Safety Board (NTSB) accident, the Federal Aviation Administration (FAA) incident, and the NASA Aviation 

Safety Reporting System (ASRS) incident datasets. In addition, future directions in aviation technology related to VHM research areas were assessed through 

the Commercial Aviation Safety Team (CAST) Safety Enhancements Reserved for Future Implementations (SERFIs), the National Transportation Safety 

Board (NTSB) Most-Wanted List and recent open safety recommendations, the National Research Council (NRC) Decadal Survey of Civil Aeronautics, and 

the Future Aviation Safety Team (FAST) areas of change. Future research direction in the VHM research areas is evidently strong as seen from recent research 

solicitations from the Naval Air Systems Command (NAVAIR), and VHM-related technologies actively being developed by aviation industry leaders, 

including GE, Boeing, Airbus, and UTC Aerospace Systems. Given the highly complex VHM systems, modifications can be made in the future so that the 

Vehicle Systems Safety Technology Project (VSST) technical challenges address inadequate maintenance crew’s trainings and skills, and the certification 

methods of such systems as recommended by the NTSB, NRC, and FAST areas of change. 

15. SUBJECT TERMS

Aviation Safety Program; Commercial Aviation; National Transportation Safety Board; Vehicle Health Management

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58

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(443) 757-5802

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